The grey heron has a slow flight, with its long neck retracted. This is characteristic of herons and bitterns, and distinguishes them from storks, cranes, and spoonbills, which extend their necks. It flies with slow wing-beats and sometimes glides for short distances. It sometimes soars, circling to considerable heights, but not as often as the stork. In spring, and occasionally in autumn, birds may soar high above the heronry and chase each other, undertake aerial manoeuvres or swoop down towards the ground. The birds often perch in trees, but spend much time on the ground, striding about or standing still for long periods with an upright stance, often on a single leg.
Fish, amphibians, small mammals, and insects are taken in shallow water with the heron's long bill. It has also been observed catching and killing juvenile birds such as ducklings, and occasionally takes birds up to the size of a water rail. It may stand motionless in the shallows, or on a rock or sandbank beside the water, waiting for prey to come within striking distance. Alternatively, it moves slowly and stealthily through the water with its body less upright than when at rest and its neck curved in an "S". It is able to straighten its neck and strike with its bill very fast.
Small fish are swallowed head first, and larger prey and eels are carried to the shore where they are subdued by being beaten on the ground or stabbed by the bill. They are then swallowed, or have hunks of flesh torn off. For prey such as small mammals and birds or ducklings, the prey is held by the neck and either drowned, suffocated, or killed by having its neck snapped with the heron's beak, before being swallowed whole. The bird regurgitates pellets of indigestible material such as fur, bones and the chitinous remains of insects. The main periods of hunting are around dawn and dusk, but it is also active at other times of day. At night it roosts in trees or on cliffs, where it tends to be gregarious.
Source : wikipedia